Vertical and Horizontal Groundwater Fluxes
at the CO2 Site

 

Hydrologic flow was investigated in the marsh in order to determine whether dissolved inorganic carbon was being exported from the marsh in groundwater as suggested by higher concentrations of porewater DIC (See figure).

Dissolved Inorganic C at ambient (open circles) and elevated (closed) CO2.

Hydraulic gradients and hydraulic conductivity were measured at a series of points that encompassed the upland and the wetland portion of Kirkpatrick Marsh where the elevated CO2 study is located. Piezometer wells were installed at two depths (30 and 75 cm) at seven locations. Hydraulic conductivity in the root zone (30cm depth) averaged 1.1x10 -3 cm/s, which indicates the potential to effectively convey water through the root zone in both a horizontal and vertical direction. However, the horizontal hydraulic gradient was small and horizontal water flow was negligible. Measurements of h ydraulic head made seasonally over two years showed that the average hydraulic gradient was 0.1%±0.3% in the horizontal dimension and 14% in the vertical dimension. These data indicate that the hydrology of the site is similar to other tidal marshes, all of which have extremely low horizontal flow (approaching zero) at distances from tidal creeks >30 m. The distance between the elevated CO2 site and the tidal creek is ~300 m.

Vertical hydraulic gradients can be created by vertical discharge of groundwater, flooding (which occurs 3% of the time at the CO2 site) or evapotranspiration. For insight on whether vertical hydraulic gradients were a result of evapotransporation or vertical groundwater upwelling, we measured porewater SO 4 content. The SO 4 content at 1 and 2 m depth was about 4 mM, which more concentrated than surface floodwater and not indicative of groundwater discharge.

In summary, in-situ hydraulic conductivity measurements indicated that horizontal fluxes at the site are minimal and that hydrologic export of carbon does not occur. Our provisional conclusion is that groundwater flow at this site is driven by evapotranspiration. Water lost by this pathway is replaced by tidal water infiltration during flood events, or pressure-driven upwelling from below in between flood events. Dissolved CO2 in the upwelling water is currently captured in our measurements of CO2 flux.


Research Questions